This invention concerns an improved apparatus and process for melt spinning uniform polymeric filaments, especially in the form of continuous filament yarns, by spinning at controlled withdrawal speeds.
It has long been known that polymeric filaments, particularly lighter denier textile filaments such as polyesters and polyamides, can be prepared directly, i.e., in the as-spun condition, without any need for drawing, by spinning at high speeds of the order of 5 km/min or more. This was first disclosed by Hebeler in U.S. Pat. No. 2,604,667 for polyesters, and by Bowling in U.S. Pat. No. 2,957,747 for polyamides. To improve process economics, there has been increased interest in the last 10 years, in melt-spinning uniform polymeric filaments without sacrificing good properties at the highest spinning speeds possible.
Frankfort et al. in U.S. Pat. Nos. 4,134,882 and 4,195,051 disclose new uniform polyester filaments and continuous filament yarns of enhanced dyeability, low boil-off shrinkage and good thermal stability, prepared by spinning and winding directly at withdrawal speeds of 5 km/min or more. The highest withdrawal speed (spinning speed) exemplified is 8000 ypm (7.2 km/min). The withdrawal speed is the speed of the first driven roll wrapped (at least partially) by the filaments, i.e., the feed roll. When uniform polymeric filaments are desired, such as are suitable for continuous filament yarns, for example, it is essential to use a roll or equivalent positive means, driven at a constant controlled speed to withdraw the filaments, as opposed to an air jet ejector. The latter is satisfactory for some uses, such as non-woven products, but does not produce filaments that are sufficiently uniform for use as continuous filament yarns for most purposes.
Tanji et al. U. S. Pat. No. 4,415,726 reviews several earlier references and disclose polyester filaments and yarns capable of being dyed under normal pressure, and a process for producing such polyester yarns with improved spinning stability at controlled high spinning (i.e., withdrawal) speeds of over 5 km/min. An important element is the subjection of the filaments to a vacuum or suction by an aspirator.
Vassilatos in U.S. Pat. No. 4,425,293 discloses an oriented amorphous polyethylene terephthalate textile feed yarn for false-twist texturing prepared by spinning polyethylene terephthalate at a speed of over 5000 m/min and quenching in a liquid bath to provide filaments having a boil off shrinkage (BOS) of at least 45% and no detectable crystallinity as measured by customary X-ray diffraction procedures. The yarn produced has a relatively low elongation to break (&lt;30%).
There has also been increased interest in improving productivity of heavier denier, e.g., industrial, yarns via increased spinning speeds without sacrificing good yarn properties. Zimmerman in U.S. Pat. No. 3,091,015 disclosed a process for spinning heavier denier (e.g., 6 to 12 dpf) industrial yarns at speeds of 440 ypm at the first feed roll to produce the desirable low birefringence yarns needed to obtain good mechanical yarn properties after the drawing steps. It would be very desirable from an economic viewpoint to provide an improved process and apparatus which will remove the spinning speed limitations or raise the plateau which presently exists in the low denier textile yarns as well as heavy denier industrial yarns without sacrificing good filament properties. However, an article by Professor A. Ziabicki in Fiber World, September 1984, pages 8-12, entitled "Physical Limits of Spinning Speed" questions whether higher speeds can yield fibers with better mechanical properties, and whether there are any natural limits to spinning speed which cannot be overcome (concentrating on physical and material factors only, and excluding economical and technical aspects of the problem). Professor Ziabicki concludes that there exists such a speed, beyond which no further improvement of structure and fiber properties is to be expected. In the case of polyester textile filaments the maxima appear to Professor Ziabicki to be around 5-7 km/min. This is consistent with the results shown by Tanji at speeds up to 9 km/min. For the heavier denier industrial yarns, although no such statement was made, no disclosure in the published literature was found which taught how to raise the spinning speed plateau for these yarns.
Furthermore, it was found that processes disclosed in the above cited references either did not allow spinning at much above the current speeds due to process discontinuity problems or to drastic deterioration of filament properties as the spinning speeds increased.
In contrast to Tanji's disclosure of preparing polymeric filaments by winding at high withdrawal speeds, with an aspirator to assist the withdrawal of the filaments from the spinneret, there have been several disclosures of preparing polymeric filaments by extruding into a pressurized chamber and using air pressure, e.g., an air nozzle or an aspirator to withdraw the filaments from the pressurized chamber without use of any winder or other positively-driven roll to advance the filaments at a controlled speed. The resulting filaments have many uses, especially in non-woven fabrics, but do not have the uniformity required for most purposes as continuous filament yarns, because of the inherent variability (along the same filament and between different filaments) that results from use of only an air jet to advance the yarns, i.e., without a winder or other controlled positive-driving mechanism. Indeed, the resulting filaments are often so non-uniform as to be spontaneously crimpable, which can be of advantage, e.g., for use in non-wovens, but is undesirable for other uses.
Accordingly, it was very surprising, according to the invention, to provide an improved process for obtaining polymeric filaments and yarns by spinning at significantly higher than conventional spinning speeds, with similar or better mechanical properties than has been shown and predicted in the prior art for both light and heavy denier yarns.